1. Field of the Invention
The present invention relates generally to resins which are used in the fabrication of panels which include a honeycomb core sandwiched between two face sheets. More particularly, the present invention is directed to the resins which are used in making the honeycomb core and/or the face sheets.
2. Description of Related Art
Honeycomb structures are well known and widely used in many applications where a high-strength and lightweight material is required. The combined features of lightweight and strength found in honeycomb structures make them particularly well suited for use in aircraft and other applications where high strength and low weight are required. Honeycomb structures have been made from a wide variety of materials including metals, such as aluminum. Composite materials made from resin impregnated fibers and papers have also been widely used in honeycomb structures.
A popular use for honeycomb structures is in the formation of structural panels where a honeycomb is sandwiched as a core material between two face sheets. A wide variety of both metallic and non-metallic materials have been used as face sheets. Composite materials made from resin impregnated fibers have been a popular material which is used widely in the formation of face sheets.
Honeycomb sandwich panels having a core and face sheets made completely with composite materials have found wide application due to their lightweight and structural strength. However, many composite panels are not particularly resistant to heat and fire. Further, upon ignition, such panels generate smoke. Accordingly, there has been a continuing effort to develop composite sandwich panels which have good heat and flame resistance and generate reduced amounts of smoke during burning.
Phenolic resins have been widely used in fabricating both the honeycomb core and face sheets. Phenolic resins are popular because they tend to be relatively flame and heat resistant as well as being relatively low in cost. In addition, phenolic resins tend to generate lesser amounts of smoke than other resins, such as epoxies, which are commonly used in fabricating sandwich panels. Examples of existing types of temperature resistant composite panels are described in U.S. Pat. Nos. 5,309,690; 4,598,007; 4,299,872; 4,557,961; 5,798,307; 6,153,687 and EPA Publication No. 0624462A1.
In accordance with the present invention, it was discovered that the amount of smoke and heat generated by a honeycomb sandwich panel during burning can be reduced by modifying phenolic resins to reduce the hydrogen/carbon ratio of the resin. The invention is based on the discovery that a low-smoke producing modified phenolic resin can be made by reacting an aldehyde with a modified phenol composition in the presence of a catalyst for a sufficient time and at a sufficient temperature to form a resole resin which has sufficient strength to make it suitable for use in making honeycomb sandwich panels.
The modified phenol composition which is used to form the final phenolic resin is made in accordance with the present invention by substituting up to 80 weight percent of a low smoke producing compound in place of phenol. The modified phenol composition includes from 20 to 80 weight percent of phenol and from 20 to 80 weight percent of a low-smoke producing compound, such as, bisphenol-S (4,4xe2x80x2-dihydroxydiphenyl sulfone), biphenol or bisphenol ethers.
The modified phenol composition exhibits a reduced level of methylene linkage formation during curing of the phenolic resin. The replacement of methylene groups by sulfone groups produces a cured resole resin that generates less smoke and heat when burned and also exhibits a reduced burn rate.
The low smoke producing resin in accordance with the present invention is suitable for use as a dip resin to coat honeycomb cores. The modified resin may also be used as the matrix resin used in fabricating the face sheets and basic honeycomb core structure. The invention has wide application to situations where high-strength sandwich panels are required and where heat resistance and low-smoke generation is required.
The above-described and many other features and attendant advantages of the present invention will become better understood by reference to the following detailed description when taken in conjunction with the accompanying drawings.